(a) Field of the Invention
The present invention relates to a display device. More particularly, the present invention relates to an organic electroluminescent display using electroluminescence (hereinafter, ‘EL’) of organic materials.
(b) Description of the Related Art
In general, an organic electroluminescent (EL) display electrically excites a phosphorous organic compound to emit light. The organic emitting elements (or organic emitting cells) are arranged in an n×m matrix format to configure an organic EL display panel which displays image data by voltage- or current-driving.
The organic emit element has diode properties, and thus is also referred to as an organic light emitting diode (OLED). The organic emit element includes an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. Further, the organic emitting element includes an electron injecting layer (EIL) and a hole injecting layer (HIL). The organic emitting elements are arranged in an n×m matrix format to configure an organic EL display panel.
Methods for driving the organic EL display panel include a passive matrix method, and an active matrix method which uses thin-film transistors (TFTs). The passive matrix method includes forming anodes and cathodes to cross (or cross over) with or to be substantially perpendicular to each other, selecting lines, and driving the organic EL display panel. The active matrix method includes orderly turning on of a plurality of TFTs which are respectively coupled to data lines and scan lines according to signals for selecting the scan lines and driving the organic EL display panel.
Hereinafter, a pixel circuit of a general active matrix organic EL display is described.
FIG. 1 shows a pixel circuit, one of n×m pixels, which is located at a first row and a first column.
As shown in FIG. 1, one pixel 10 includes three sub pixels 10r, 10g, and 10b, which respectively include organic EL elements OLEDr, OLEDg, and OLEDb for respectively emitting a red light (R), a green light (G), and a blue light (B). Further, in the structure the sub pixels are arranged in a stripe format, and the sub pixels 10r, 10g, and 10b are coupled to separate data lines D1r, D1g, and D1b and to a common scan line S1.
The red color sub pixel 10r includes two transistors M11r and M12r and a capacitor C1r for driving the organic EL element OLEDr. In the same manner, the green color sub pixel 10g includes two transistors M11g and M12g and a capacitor C1g for driving the organic EL element OLEDg, and the blue color sub pixel 10b includes two transistors M11b and M12b and a capacitor C1b for driving the organic EL element OLEDb. Since the connections and operations of the sub pixels 10r, 10g, and 10b are substantially the same, only the connection and operation of the sub pixel 10r will now be described as an example.
The driving transistor M11r is coupled between a power supply source voltage VDD and an anode of the organic EL element OLEDr, and transmits a current for emitting light to the organic EL element OLEDr. A cathode of the organic EL element OLEDr is coupled to a voltage VSS which is lower than the power supply source voltage VDD. The amount of the current flowing in the driving transistor M11r is controlled by a data voltage applied through the switching transistor M12r. The capacitor C1r is coupled between a source and a gate of the transistor M11r and controls applied voltage during a predetermined period. A scan line S1 for transmitting an on/off selection signal is coupled to a gate of the transistor M12r, and a data line D1r for transmitting a data voltage corresponding to the red color sub pixel 10r is coupled to a source of the transistor M12r.
Here, the switching transistor M12r is turned on in response to a select signal which is applied to the gate. Then, the data voltage VDATA is applied to the gate of the transistor M11r from the data line D1r through the transistor M12r. Then, a current IOLED flows to (and/or through) the transistor M11r corresponding to a voltage VGS which is charged between the gate and source of the transistor M11r by the capacitor C1r. The organic EL element OLEDr emits red light corresponding to the current IOLED. The current flowing to the organic EL element OLEDr is calculated as given in the following Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, VTH is a threshold voltage of the transistor M11r, and β is a constant.
As represented by Equation 1, a current corresponding to a data voltage is supplied to an organic EL element OLEDr in the pixel circuit shown in FIG. 1, and the organic EL element OLEDr emits a red light to brightness corresponding to the supplied current. Here, the supplied data voltage has multistage voltage values in a predetermined range to display a certain gray scale.
As such, in the organic EL display, one pixel 10 includes three subpixels 10r, 10g, and 10b, and each sub pixel includes a driving transistor, M11r, M11gor M11b, a switching transistor, M12r, M12g or M12b, and a capacitor, C1r, C1g or C1b, for driving the organic EL element, OLEDr, OLEDg or OLEDb. Further, each sub pixel is coupled to a data line for transmitting a data signal and a power line for transmitting a power supply source voltage VDD.
Therefore, many lines for transmitting voltages and signals to a transistor and a capacitor are required to be located in each pixel, and there is difficulty to arrange all the lines within one pixel.